How to set volume in xubuntu?

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http://askubuntu.com – I want to set via my Media Keys, so I first have to find out the terminal-command. Outputs are: amixer set Master 10%- amixer: Unable to find simple control 'Master',0 amixer scontrols Simple mixer control 'IEC958',0 amixer set 'IEC958' 50%- amixer: Invalid command! How can i fix that? (HowTos)